1. Field of the Invention
This invention relates to a process for precipitating aluminum trihydroxide from Bayer process liquors, and more particularly to improvements in the Bayer process which help to increase the yield of alumina and the size of alumina particles retrieved from Bayer process liquors. The invention also relates to improved apparatus for increasing the yield and size of alumina particles obtained in the Bayer process.
2. Description of the Prior Art
Various attempts have been made to increase the yield and particle size of the alumina recovered from Bayer process liquors in the more than 100 years since the invention of the Bayer process.
Recent attempts at improvements have included modifying various aspects of the process including dividing the incoming pregnant liquor stream into more than one part and supplying one part to a series of tanks known as the agglomeration section, and the remainder of the stream to a second series of tanks known as the growth section.
For example, a process described in U.S. Pat. No. 4,311,486 (Yamada) divides the incoming flow into two portions, and adds from 30 to 150 grams/liter of seed having a particle size of less than 10 microns to the agglomeration section, and from 30 to 150 grams/liter of coarse seed wherein at least 10% by weight of grains passing through a 325-mesh Tyler sieve is used in the growth section.
Another approach, contained in U.S. Pat. No. 4,614,642 (Cristol), also involves dividing the incoming pregnant liquor into two parts. According to that patent, 700 grams/liter of ungraded seed should be added to the agglomeration stage, and no seed should be added to the growth stage of the process.
Still other efforts have been made to improve yield and particle size in the Bayer process. U.S. Pat. No. 4,305,913 (Anjier), U.S. Pat. No. 4,234,559 (Tschamper), and U.S. Pat. No. 4,617,179 (Veyrier), each disclose attempts to solve the foregoing problems based upon adjustments to the various steps in the Bayer process.
None of the foregoing techniques, however, has been entirely successful in increasing yield and particle size of the alumina recovered from the Bayer process liquors.